Neuropsychiatric disorders are the leading cause of disease burden in the United States and Canada, far outpacing other maladies such as cardiovascular disease and cancer (WHO statistics). Progress in treating neuropsychiatric disorders is severely hampered by our lack of basic knowledge related to their underlying causes. Defects in dendritic spine morphogenesis, a process regulated by dynamic actin remodeling, is a common feature of these disorders and is also associated with stress, which may precipitate disorders such as schizophrenia. Moreover, it is increasingly clear that disruptions in genes that regulate signaling to excitatory synaptic actin are risk factors for schizophrenia, autism, and intellectual disability. Arp2/3 complex is enriched in dendritic spines and stimulates the formation of branched actin downstream of many genes implicated neuropsychiatric disorders. Recently we published that the conditional loss of Arp2/3 in mice leads to the progressive development of multiple synaptic and behavioral phenotypes relevant to models of schizophrenia. Many of the schizophrenia-related behaviors are normalized by the antipsychotics clozapine and haloperidol. The specific aims of this grant build on these exciting findings to address fundamental questions of how SZ-related phenotypes evolve at the synaptic and circuit level and how this is influenced by chronic stress. We anticipate the results of these aims will bridge our knowledge gap regarding how SZ-like phenotypes emerge in vivo, leading to new future directions for the prevention and possible treatments of the disorder.